The present invention pertains to the platform scale art and, more particularly, to an improved platform scale of the type having strain gauge load sensors mounted to a deflectable load scale.
Platform scales for use in measuring the axle load of vehicles, such as commercial load carrying trucks, are well known to the art.
In one commonly found platform scale construction, the wheels of the vehicle are positioned on a weighbridge platform. The load on the platform is transmitted to a load cell housed within a tray mounted beneath the platform. The load cell employs a beam element that is supported at either end with respect to the tray. The load on the platform is transmitted to the central portion of the beam, producing a rotational displacement of the beam about its tray supports. The deflection or shear displacement of the beam is monitored by one or more strain gauge transducers affixed thereto. The transducers produce output electrical signals which may be suitably summed and scaled for application to an electronic readout, indicating the total weight on the platform.
Each wheel or wheel group of a vehicle may be sequentially weighed in this manner or, in the alternative, multiple weighbridge platforms may be provided, such that all wheels are positioned on weighbridge platforms simultaneously.
In one particular prior art platform scale assembly, an elongate load cell has semicylindrical grooves provided in the undersurface of each end thereof. The grooves are positioned on bearing rods that ride on bearing plates affixed to the floor plate of the load cell tray. A weighbridge platform is affixed to the load cell and tray assembly such that a load on the bridge is transmitted to the load cell at a portion intermediate the bearing rods, thereby creating a rotational displacement of the load cell about the bearing rod pivot points. This displacement produces an output signal on bending moment strain gauge transducers affixed to the undersurface of the load cell.
To prevent the load cell from being lifted out of its tray, a pair of retaining plates are affixed to the top surface of either end of the load cell tray. The retaining plates are positioned such that either end of the load cell, upon being lifted off of its pivot, is constrained by its corresponding retaining plate.
The aforedescribed vehicle platform scale assembly is subject to an inaccuracy when a vehicle is positioned at one edge of the platform. Such an off-centered loading may result in a moment being applied to the load cell causing tilting thereof, with one end of the load cell engaging its retaining plate. In this event, the readings from the strain gauges may not accurately reflect the vehicle load on the platform due to three factors. First, a tilt of the load cell's longitudinal axis away from the horizontal plane introduces a "cosine .theta." error. That is, if the load cell deviates by an angle .theta. from the horizontal, its deflection in response to a transmitted load is reduced by a factor related to cosine .theta.. This reduced deflection, as sensed by the strain gauge transducers, produces a reading less than the actual weight of the load.
A second source of error produced by a tilt of the load cell within its tray such that one end thereof bears on its retaining plate results from a repositioning of the load cell pivot points and corresponding beam length variances. That is, the load cell is calibrated using a fixed load which produces a corresponding deflection of the load cell about its bearing rod pivot points. The resulting signal produced by each bending moment strain gauge transducer is a function of the beam length from the transducer mounting point to the load cell pivot point. For the condition wherein one end of the load cell is lifted off of its bearing rod pivot and contacts the underside of the retaining plate, and is now pivoting about some indeterminate point on the retaining plate, the beam length changes, producing a corresponding change in the transducer output.
A third source of inaccuracy caused by tilt of the aforedescribed load cell within its tray is due to the asymmetrical configuration of the load cell in its vertical dimension. As a result of this asymmetry, the neutral axis of the load cell moves when the pivot point thereof displaces from the underside to the top surface of the load cell. Such a skewing of the load cell's neutral axis produces a change in the stress levels at the locations of the strain gauge transducers and, correspondingly, introduces an error into the transducer-produced signals.